Does This Dinosaur Make Me Look Fat?

Body mass is probably the most important physiological features for all animals. It corresponds strongly with a range of life features, including metabolic and growth rates, population density, diet and dietary strategy, locomotion style and mechanics, and mode of reproduction. It comes as perhaps no surprise then that body mass is one of the most widely explored features of extinct organisms by paleontologists. Last year, a slew of papers explored the evolution of body size in dinosaurs, including birds.

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Body Mass

Body mass is probably the most important physiological features for all animals.

It corresponds strongly with a range of life features, including metabolic and growth rates, population density, diet and dietary strategy, locomotion style and mechanics, and mode of reproduction.

It comes as perhaps no surprise then that body mass is one of the most widely explored features of extinct organisms by paleontologists. Last year, a slew of papers explored the evolution of body size in dinosaurs, including birds (e.g., this one in PLOS Biology). Most of these found that rapid changes in maniraptoran theropods, the dinosaurian lineage leading to modern birds, occurred from the Middle Jurassic (about 160 million years ago) and onwards.

Body Evolution

Different rates of body size evolution across dinosaurs (including birds).

Image by Benson et al (2014)

Adapting To Change

Importantly, this means that, in terms of body size, birds were constantly and rapidly innovating and changing, which might have set the scene for the origins of the great bird radiation.

It’s weird to think about, but with 10,000 living species of bird, we are still technically in the ‘reign of the dinosaurs’, and it might be due to an early ability to rapidly evolve body size and adapt to changing conditions.

In birds, body mass has one additional and unique factor in that it correlates with the amount of lift an animal can generate, and therefore influences whether or not they can fly! Therefore, being able to accurately estimate body mass in extinct birds has important implications for our understanding of the origins of flight.

Trusting Estimations

Previous studies, including those mentioned above, have had to rely on proxies to estimate body mass.

It’s ridiculously unlikely that we’ll ever find a complete dinosaur, and we only have their skeletons to go off. One way of estimating body mass has been to use the circumference of the femur, which correlates strongly with body mass in a range of living organisms – known as an ‘allometric’ relationship.

Estimates of body mass in birds have also been applied to pterosaurs, a group of now extinct flying reptiles related to dinosaurs. But the question remains, how accurate are our estimations of body mass in the fossil record?

Figure 1

Linear scale association between total body mass and skeletal mass in birds.

Positive Associations

A new study, led by Liz Martin-Silverstone at the University of Southampton in the UK, set out to divine the relationships between skeletal mass and complete or total body mass in birds (i.e., involving all the fleshy parts).

What they found, using a range of analyses and datasets, was a strong positive association between body mass and skeletal mass, as we might expect – as the skeleton of an animal gets bigger, so does its overall mass. This is important, as it means that for living neornithine birds (at least), estimates of skeletal mass accurately reflect total body mass, and therefore skeletal mass can be used as a proxy to estimate the life traits mentioned at the beginning of this post.

Figure 2

Logarithmic scale association between total body mass and skeletal mass in birds.

Natural Variation

Despite overall good correlations, the authors found quite a lot of natural variation within species, based on an extensive new dataset compiled from the collections at the Royal British Colombia Museum (Victoria, Canada).

This is simply due to the fact that we have different animals of different sizes within species – take a look at humans, for just one obvious example of this. An example from birds is the rhinoceros auklet, which has a total body mass ranging from 258-616.2 grams!

Figure 3

The Reason Behind Variations

The reason for such variation can also be due to age – it’s a pretty well established phenomena that animals get bigger as they grow up.

This has drastically important implications for estimating skeletal mass across animals in the fossil record. For each animal, they would have to be shown to be the same growth stage, or ontogenetic age, so that their body masses could be directly comparable.

There’s not really much point comparing the body mass of a juvenile of one species to that for a fully grown individual of another! Birds also grow ridiculously fast (when was the last time you saw a baby pigeon?), so it can be very difficult to accurately tell what their ages are without detailed examination.

Identifying Cofounding Factors

The authors also identified a range of confounding factors that influence estimates of body mass.

For example, when female birds are ready to lay eggs, they accumulate and deposit more calcium within their bones to save it for egg production. So we might expect the skeletal mass to vary between males and females of the same species, depending on sexual maturity.

However, there were no significant differences between the sexes, despite this possible variation. As well as this, migratory birds have very different weights before and during migrations, although this is relatively slight at just a few percent difference, but whether or not this affected the results is unknown.

Analyzing Flight Mode

What about flight mode?

Does this affect estimates of body mass, as we might expect flight capable birds to have bigger muscles for flapping their wings, or perhaps be lighter in order to generate more lift for flight. Martin-Silverstone and colleagues found, however, that there was again no statistical difference in the relationship between body mass and skeletal mass across different flight modes.

Does This Dinosaur Make Me Look Fat?

It’s Difficult To Infer

This is great, as in fossil birds, it suggests that even if we don’t know their flight style, as it’s notoriously difficult to infer in extinct animals, we can still accurately estimate their body mass.

The authors are careful to note though that their analyses did not cover all birds, and seems to have excluded a whole range including penguins, ratites (kiwis, emus, and ostriches), cormorants and a whole load of other avian weirdos.